In all protein variants, donor label (i.e., A488) is attached to residue 69. (a) Positions of dye labels within the topology of flavodoxin. A488 is represented by a bright green star and acceptor label (i.e., A568) by a pink star. (b) Cartoon representation of d69-a1, showing in blue the backbone that intervenes residues 1 and 69. (c) d69-a131, with the backbone intervening residues 69 and 131 in green. (d) d69-a178, with the backbone intervening residues 69 and 178 in orange. A488 is shown in bright green, and A568 in pink. Cartoons are generated with PyMOL (Schrödinger, LLC, Palo Alto, Ca, USA) using the crystal structure of A. vinelandii flavodoxin (pdb ID 1YOB [56]) and the molecular structures of A488 and A568, as provided by Invitrogen.